1. Field of the Invention
The present invention relates to methods for enhancing the performance of disk drives through thermal monitoring. More particularly, the present invention relates to improved methods for controlling a voice coil motor (VCM) adapted for moving a head over a disk, to prevent overheating of a coil in the VCM due to excessive application of current to the coil.
2. Description of the Prior Art and Related Information
A typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board assembly (PCBA). The HDA includes at least one magnetic disk (disk), a spindle motor for rotating the disk, and a head stack assembly (HSA) that includes a read/write head with at least one transducer for reading and writing data. The HSA is controllably positioned by a servo system in order to read or write information from or to particular tracks on the disk. The typical HSA has three primary portions: (1) an actuator assembly that moves in response to the servo control system; (2) a head gimbal assembly (HGA) that extends from the actuator assembly and biases the head toward the disk; and (3) a flex cable assembly that provides an electrical interconnect with minimal constraint on movement.
A xe2x80x9crotaryxe2x80x9d or xe2x80x9cswing-typexe2x80x9d actuator assembly comprises a body portion that rotates on a pivot bearing cartridge between limited positions, a coil portion that extends from one side of the body portion to interact with one or more permanent magnets to form a VCM, and an actuator arm that extends from an opposite side of the body portion to support the HGA.
Within the HDA, the spindle motor rotates the disk or disks, which are the media to and from which the data signals are transmitted via the read write/head(s) on the gimbal attached to the load beam. The performance of the disk is largely dominated by its mechanical latencies. One such mechanical latency is the rotational latency of the drive, which is a function of rotational speed of the disk and hence of the spindle motor. Another such mechanical latency is the seek latency of the drive, which is a function of the speed at which the actuator radially moves across the disk.
Competitive pressures in the disk drive market have compelled disk drive designers and manufacturers to simultaneously boost performance and lower cost. Historically, higher performance has been achieved by, for example, increasing the rotational speed of the spindle motor and/or performing faster seek operations. Faster seek operations, in turn, may be achieved by increasing the control current flowing through the VCM, thereby increasing the actuator""s acceleration and deceleration as it moves across the disk. Excessive VCM control currents or control current profiles having a high average value however, may cause the VCM assembly (typically overmolded with a plastic material) to overheat, causing damage to the coil and the drive. For example, when subjected to an instantaneous or average current that is beyond the VCM""s design limitations, the coil may generate excessive heat and rupture, or the coil overmold material may delaminate from the actuator assembly, lose its rigidity and/or outgas particulates into the disk drive enclosure, with deleterious results. Such outgassing from the coil overmold, coil insulators and/or from other materials applied to the coil wires (such as wire lubricants, for example) may occur even at relatively low temperatures (85xc2x0 C., for example). There is a need, therefore, to monitor the temperature of the VCM coil and to prevent damage thereto.
One possible solution that addresses the need to prevent excessive VCM temperatures is to limit the VCM control current so that the heat generated therein remains at all times within conservative limits, independent of present actuator usage patterns. This solution, while effectively preventing the VCM from overheating and obviating the need to monitor the temperature thereof, also results in unacceptably slow drive performance. Another solution is proposed in U.S. Pat. No. 5,594,603 to Mori et al. and assigned to Fujitsu Limited, Japan. In this patent, the current applied to the VCM is used to calculate an approximation of the VCM temperature. This method attempts to mathematically model the thermal behavior of the VCM by devising a number of predetermined coefficients and by quantifying and inter-relating the VCM control current, the heat naturally radiated by the VCM, the ambient temperature, the thermal capacity of the VCM and the ambient temperature thereof, among other factors. However, such a mathematical model, although providing an indication of the present VCM temperature, may not accurately provide a calculated temperature value that accords with the present and actual temperature of the VCM. Indeed, a number of factors may skew the results obtained from such mathematical models. For example, the present temperature of the drive or the resistance of the VCM coil may not remain constant and may thus result in changing VCM control current magnitudes. As the VCM control current is used as the basis for the temperature calculations, the VCM may not be driven (i.e., supplied with VCM control current) in an optimal manner and the actuator may not sweep as rapidly across the disk as it might otherwise have, thereby needlessly limiting the overall performance of the drive. Alternatively, should the mathematical model prove to be an inaccurate predictor of actual VCM temperature in certain situations, excessive VCM control currents may be generated, potentially causing damage to the VCM and to the drive. Over many iterations, recursively-applied mathematical models may cause an initial and relatively small error to grow to such a degree that the model no longer accurately reflects present operating conditions. Reliance upon such an inexact mathematical model in modulating the VCM control current may understandably result in less than optimal drive performance characteristics.
Another proposed solution is proposed in U.S. Pat. No. 5,594,603 to Lee (hereafter the ""603 patent) and assigned to Quantum Corporation. In this patent, a discrete temperature-sensing element is used to dynamically sense the VCM temperature during the operation of the drive. The output of the temperature-sensing element (e.g., thermistor) is quantized and used to calculate a multiplication factor. The multiplication factor, in turn, is multiplied by a reference velocity command during a seek operation to produce a velocity command that is then compared with a fed back velocity value to generate an error signal that modulates the operation of the actuator (e.g., the VCM control current) during track seek operations. This patent discloses that the thermistor is mounted for thermal conduction directly to the head and disk assembly. While the temperature sensing element may, in fact, provide a direct measurement of the temperature of the VCM (in contrast to the Mori et al. patent above, for example), this method requires mounting a high precision thermistor to the HDA and requires that appropriate signal conditioning means be provided to measure, quantize and interpret the resistance thereof. In many aspects, however, disk drive designers and manufacturers operate in an environment that has acquired many of the characteristics of a commodity market. In such a market, the addition of even a single, inexpensive part can directly and adversely affect competitiveness. In this case, therefore, the addition of the thermistor and associated signal conditioning means discussed in the ""603 patent may be of little practical value.
What are needed, therefore, are methods for monitoring the temperature of a disk drive voice coil motor that are accurate, reliable and inexpensive in their implementation. Specifically, what are also needed are temperature monitoring methods, disk drives and computer systems that do not rely upon complex and error prone mathematical modeling schemes or upon costly temperature sensing circuitry. Also needed are methods for monitoring the VCM temperature that allow the VCM control current to be modulated in an optimal manner to achieve fast track seek operations.
An object of this invention, therefore, is to provide accurate, reliable and inexpensive temperature monitoring methods, as well as disk drives implementing such methods. Another object of this invention is to provide methods for monitoring the VCM temperature that allow the VCM current to be modulated in an optimal manner to prevent overheating of the coil and to achieve fast seek operations.
Accordingly, this invention can be regarded as a method for controlling a VCM adapted for moving a head over a disk, to prevent overheating of a coil in the VCM due to excessive application of current to the coil. According to an embodiment thereof, the method of the present invention includes the steps of defining an acceleration interval (T) and a reference distance (D1). Current is the applied to the coil during a seek operation, thereby causing a rise in coil temperature. Current is then applied to the coil during the acceleration interval and the distance traveled (D2) by the head during the acceleration interval is the measured. The distance traveled during the acceleration interval is compared to the reference distance to obtain a difference (D2xe2x88x92D1). If the difference exceeds a threshold (e.g., D2 less than D1, for example) the current being applied to the coil is controlled to maintain the coil temperature within a limit. If, however, the difference does not exceed the threshold, the current being applied to the coil is controlled to meet a performance requirement.
According to further embodiments, the acceleration interval may be defined as a function of a number of servo wedges traversed by the head as the disk rotates at constant angular velocity. The reference distance defining step and the distance traveled measuring step may be determined as a function of a number of tracks of the disk swept by the head during the acceleration interval. The steps of applying current during the acceleration interval, measuring and comparing may be carried out independently of the seek operation and/or at a predetermined interval. The predetermined interval may be adaptive, the interval being shorter during periods of high disk accesses and lower during periods of lower disk accesses. Alternatively, the steps of applying current during the acceleration interval, measuring and comparing may be carried out incident to the seek operation. The step of controlling the current being applied to the coil to maintain the coil temperature within a limit may include a step of reducing the current being applied to the coil. The limit may be selected as the highest temperature at which the coil does not overheat. For example, the limit may be selected to be about 85xc2x0 C. The distance traveled and/or the difference may be adjusted to compensate for changes in a coil resistance and/or voltage. The distance traveled and/or the difference may be discounted to compensate for an increased coil resistance and/or voltage. The reference distance may be defined at a known temperature, such as test chamber temperature, for example.
The present invention may also be viewed as a disk drive which, according to one embodiment, includes a disk having a recording surface for recording data thereon; a microprocessor for controlling an operation of the disk drive; a servo controller controlled by the microprocessor and an actuator including a head to read and write data on the disk; a VCM controlled by the servo controller, the VCM causing the head to sweep radially across the disk in response to a VCM control current and a VCM driver, controlled by the servo driver, for applying the VCM control current to the VCM. A plurality of processes are spawned by the microprocessor, the processes including processing logic for controlling the VCM to prevent overheating of a coil in the VCM due to excessive application of VCM control current to the coil. The processing logic includes or causes the following steps to be carried out: defining an acceleration interval (T); defining a reference distance (D1); applying VCM control current to the coil during a seek operation to cause a rise in coil temperature; applying VCM control current to the coil during the acceleration interval; measuring a distance traveled (D2) by the head during the acceleration interval; comparing the distance traveled during the acceleration interval to the reference distance to obtain a difference (D2xe2x88x92D1); and if the difference exceeds a threshold (D2 less than D1): controlling the VCM control current being applied to the coil to maintain the coil temperature within a limit; else if the difference does not exceed the threshold: controlling the VCM control current being applied to the coil to meet a performance requirement.
According to further embodiments, the acceleration interval may be defined as a function of a number of servo wedges traversed by the head as the disk rotates at constant angular velocity. The reference distance defining step and the distance traveled measuring step may be determined as a function of a number of tracks of the disk traversed by the head during the acceleration interval. The steps of applying current during the acceleration interval, measuring and comparing may be carried out independently of the seek operation and/or at a predetermined interval. The predetermined interval may be adaptive, the interval being shorter during periods of high disk accesses and lower during periods of lower disk accesses. Alternatively, the steps of applying current during the acceleration interval, measuring and comparing may be carried out incident to the seek operation. The step of controlling the current being applied to the coil to maintain the coil temperature within a limit may include a step of reducing the current being applied to the coil. The limit may be selected as the highest temperature at which the coil does not overheat. For example, the limit may be selected to be about 85xc2x0 C. The distance traveled and/or the difference may be adjusted to compensate for changes in a supply voltage or a fixed voltage may be provided. The reference distance may be defined at a known temperature, such as test chamber temperature, for example.
The foregoing and other features of the invention are described in detail below and set forth in the appended claims.